a first structure;

a second structure spaced from the first structure;

an array of projections disposed between the first and second structures, each projection of the array of projections extending from the first structure toward the second structure to define a respective gap between the projection and the second structure; and

an actuator configured to move the first structure, the second structure, or the array of projections for collective adjustment of the respective gaps between the projections and the second structure;

wherein each projection of the array of projections is configured to define one or more respective unit cells, each unit cell having a sub-wavelength size relative to the acoustic beam to establish an effective refractive index profile for the acoustic beam propagating through the acoustic lens between the first and second structures, and

wherein the actuator is configured such that the collective adjustment of the respective gaps varies across the array of projections to spatially modify the effective refractive index profile for the acoustic beam to steer the acoustic beam.